Group B streptococcus (GBS), a common bacterium present in 30-40% of adult healthy women gastrointestinal and reproductive organs is transferred to infants during the passage through the birth canal. Even after extensive preventive screening and treatment of expecting mothers, GBS infects a considerable percentage of neonates, and they suffer from septicemia, pneumonia or meningitis, which are associated with high mortality. A reliable vaccine for GBS, an attractive possibility, is not ready for many more years. The complement system is a major effector arm of innate and adaptive immunity against invading pathogens. Upon activation, either it rapidly opsonizes microbes with C3b/C4b for phagocytosis or eliminate pathogens by lysing their cell walls using membrane attack complex. A credible explanation of how GBS escapes from the human complement will provide the needed input for a therapeutic intervention. Many pathogens survive the hostile host complement through sophisticated complement-modulating strategies. Some of them sequester complement regulatory molecules thereby down regulate the opsonization while others mimic complement players and many more recruit extracellular matrix molecules like fibrinogen and fibronectin, which act as bridge molecules for subsequent pathogenic uptake. In this proposal, we will characterize GBS surface proteins BibA and SHT, which subvert the complement by associating with regulatory proteins C4BP and factor fH, respectively and AvA protein that is equipped with an unusual thioester domain, similar to the complement C3b and C4b proteins. We acquired a significant amount of preliminary data in characterizing GBS BibA, SHT and AvA proteins and their respective host targets. In Aim 1, we localized the GBS BibA binding sites on complement regulator C4BP. We will determine the crystal structures of BibA and C4BP and a complex of their minimal segments. Our goal is to explain the BibA disruption of C4BP role in dissociating the classical pathway C3-convertase C4bC2a. In Aim2, we identified the minimum binding region of GBS SHT required for binding to factor H (fH). We will crystallize SHT fragments and will demarcate fH CCP domains that are essential for association with SHT. We will characterize the structure of SHTfH complex. We will identify the minimum peptide regions of BibA and SHT and the corresponding C4BP and fH, respectively that can be used for modulating the complement. These peptides or their mimics can be used as diagnostic tools for the identification of GBS and also for disruption of the host-pathogen interactions. In Aim 3, we identified the N-terminal segment of GBS AvA having a buried thioester bond (BTB) and having affinity to fibrinogen. We will characterize the association of AvA and fibrinogen by various biophysical methods. We will identify homologous proteins in other gram- positive bacteria and will design probes that can interact and broadcast the presence of BTB in them.